ScrollView: Use scrollbar height as bottom padding, not width

Summary:
Apparently I accidentally used the bottom scrollbar width instead of
height. Oops.

Test Plan: ScrollView with horizontal scrollbar no longer hides content

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D27280

(cherry picked from commit 6995b4ae81a2f3ca3c24d2fa6d1560bfe0898737)